The present invention relates to a method for manufacturing a semiconductor memory device having a high resistance to radiation such as .alpha.-rays.
Recently, metal-oxide semiconductor (MOS) memory cells such as one-transistor one-capacitor type memory cells have been developed so as to reduce the size of the elements of each memory cell, thereby obtaining a large capacity, highly integrated semiconductor memory device. However, this size reduction of the memory cells increases the generation rate of soft errors due to .alpha.-rays.
In order to reduce the generation of soft errors, various kinds of technology have been introduced. For example, technology for improving the configuration of capacitors of the memory cells has been introduced into a design step so as to effectively increase the capacitance of the capacitors. In addition, an intrinsic gettering technology has been introduced into a manufacturing step, and formation of a mechanical structure for shielding the memory cells from .alpha.-rays has been introduced into an assembly step.
The present invention is directed to the improvement of such a mechanical shield structure formed during the assembly step.
A first prior art method for forming an .alpha.-ray shield structure has been performed by a potting technology using liquid polyimide, thereby forming a polyimide layer for shielding a semiconductor chip from .alpha.-rays. In this first prior art method, however, the center portion of the polyimide layer is relatively thick so as to completely shield the chip from .alpha.-rays, while the peripheral portions of the polyimide layer are relatively thin giving an insufficient shielding. (see: GB-A-2036428)
A second prior art method is performed by using a polyimide tape having a high resistance to .alpha.-rays. That is, this polyimide tape is adhered by an adhesive to a semiconductor chip. In this case, the adhesive comprises a polyamic acid intermediate of the general formula: ##STR1## which is derived from for example pyromellitic dianhydride, as in the formula below, ##STR2## and diamine, as in the following formula EQU H.sub.2 N--R--NH.sub.2
That is, the following cyclic condensation polymerization is performed upon the pyromellitic dianhydride and the diamine so as to form heat-resistant polyimide: ##STR3##
The first stage stage reaction is reversible, while the second stage reaction is irreversible.
However, in the second prior art method, the adhering of the polyimide tape is carried out at a relatively low temperature, from about 150.degree. C. to 250.degree. C., for a relatively long time period of about 10 sec to 30 sec, and accordingly, the adhesive develops enormous macro-molecules, so that the heat shrinkage distortion of the hardened adhesive becomes large. As a result, the passivation films, the connection layers, and the like may be peeled off, thereby reducing the manufacturing efficiency and yields.